We have designed a series of studies to further evaluate the liposome as an in vivo drug delivery technique to alter the pharmacokinetics of drugs and thereby change the drug's therapeutic index. We will optimize liposomes as carriers of lipophilic prodrugs of 6-mercaptopurine in order to overcome metabolic resistance to this drug. We will identify which major liposome variable contributes most to changes in disposition on association with various tissues. We will identify the various mechanisms which govern the dose dose dependent disposition of liposomes and clarify the degree to which critical plasma proteins control liposome disposition. We will improve further our techniques to prepare liposomes of defined size. The possibility of designing light sensitive, photolytic liposomes.